test_import_from_pageserver_small: try to make less flaky

[koivunej]

With #7828 and proper fullbackup testing the test became flaky (evidence).

• produce better assertion messages in assert_pageserver_backups_equal
• use read only endpoint to confirm the row count

[github-actions]

3108 tests run: 2981 passed, 0 failed, 127 skipped (full report)

---

Flaky tests (3)

Postgres 16

• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Postgres 14

• test_download_remote_layers_api: release
• test_location_conf_churn[1]: release

Code coverage* (full report)

• functions: 31.4% (6449 of 20544 functions)
• lines: 48.3% (49854 of 103260 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
334cac7 at 2024-05-23T09:30:57.428Z :recycle:}

[koivunej]

There might be an out-of-disk situation with the arm runners which is why that step fails